The invention is generally in the field of methods and compositions for treating neurodegenerative diseases such as Parkinson's disease, alzheimer's disease, epilepsy, and the like, using viral and non-viral delivery systems that deliver therapeutic agents to specific regions of the brain. More specifically, using an adeno-associated viral vector to deliver a nucleotide sequence encoding glutamic acid decarboxylase (GAD) to specific regions of the brain that are overstimulated or disinhibited in neurodegenerative diseases.
The major inhibitory neurotransmitter in the brain is gamma-aminobutyric acid (GABA), (Roberts et al, GABA in Nervous System Function, Raven Press: New York, 1976; McGeer E G, et al, Glutamine, Glutamate, and GABA in the Central Nervous System; Hertz L, Kvamme E, McGeer E G, Schousbal A, eds., Liss: New York, 1983; 3-17). Loss of GABA signaling, by a reduction in release, loss of neurons which synthesize GABA, or antagonism of GABA receptors leads to disinhibition, overexcitation and depending on the specific brain region involved, may result in epilepsy, movement disorders or other neurological deficits and symptoms.
Diseases such as Parkinson's disease, Huntington's disease, Amyotrophic Lateral Sclerosis (ALS or Lou Gehrig's Disease), Epilepsy and Alzheimer's disease, have proved difficult to treat. Few, if any therapies, have proved effective in slowing or arresting the degenerative process associated with these diseases.
In Parkinson's Disease (PD), the primary neurochemical disturbance is believed to be the loss of substantia nigra (SN) dopaminergic (DA) neurons. This loss of DA neurons leads to a profound deficit of DA in the projection areas of the caudate and putamen and results in a loss of signaling through dopamine receptors in the postsynaptic neurons. These neurons, via efferents referred to as the direct and indirect pathways, synapse on other cells in the basal ganglia circuitry. Of most relevance to PD, the loss of dopamine receptors in the basal ganglia circuitry leads to loss of drive in the GABAergic inhibitory input to the subthalamic nucleus.
The loss of inhibitory GABAergic drive to the subthalamic nucleus (STN) results in increased activity of the STN which sends excitatory (glutamatergic) afferents to the ventrial media (VM) thalamus, the substantia nigra pars reticulata (SNPR) and a lesser projection to the pars compacta, as well as other cells within the basal ganglia including the globus pallidus. When the concentration of GABA diminishes below a threshold level in the brain, movement disorders and convulsions may result (See e.g., Karlsson et al, (1974) Biochem. Pharmacol 23:3053-3061). GABA synthesis is regulated by glutamic acid decarboxylase (GAD). GAD is present in the brain as two isoforms, GAD-65 and GAD-67. When the GABA levels rise in the brain the convulsions terminate (See e.g., Hayashi (1959) Physiol. 145:570-578). In convulsive disorders, the reduction in brain GABA levels is often paralleled by a diminished level of GAD (McGeer, et al. GABA in Nervous System Function; Roberts E, Chase T N, Tower D B, eds., Raven Press: New York 1976:487-495; Butterworth et al. (1983) Neurochem. 41:440-447; Spokes et al. (1978) Adv. Exp. Med. Biol. 123:461-473).
Levodopa (L-dopa) has historically been the medication of choice to treat Parkinson's disease. L-dopa is a precursor to dopamine and is able to cross the blood-brain barrier to target the brain. Unfortunately, the response with L-dopa is not sustainable. Most patients develop adverse effects after long-term usage of L-dopa, and often the benefits of treatment wane as the disease progresses.
Other methods for treating Parkinson's disease include transplantation of cells used to repair regions of the brain damaged by neurodegeneration. These cells can be engineered to secrete neuroactive substances such as L-dopa. The procedure typically involves cell transplantation into the stratium. However, cell transplantation is a complicated procedure which requires donor tissue, and there have been reports of mortality associated with this procedure.
Alternative forms of treating Parkinson's disease involve implanting devices for deep-brain stimulation (DBS) in specific regions of the brain. For example, DBS of the STN. These devices are typically electrodes implanted into the STN. The electrode is then stimulated at a desired frequency to reduce the effect of Parkinson's disease. The significance of the STN overactivity is reflected in the success of ablative surgery of the STN in both animal models of Parkinson's disease, as well as in human Parkinson's disease itself. In addition to ablation, implantation of nedtronic stimulators are commonly employed. The mechanism of the stimulators is believed to be mediated by local inhibition (via GABA signaling), and is replicated by the local infusion of GABA agonists.
Like Parkinson's disease, methods for treating epilepsy include the use of anti-epileptic drugs, such as sodium valporate (Epilim). Available drugs reduce seizure frequency in the majority of patients, but it is estimated that only about forty percent are free of seizures despite optimal treatment. Other forms of treatment include DBS of certain regions of the brain, such as the VIM (ventral intermediate thalamus), subthalamic nucleus, and internal globus pallidus. However, the DBS procedure is not always effective in many patients who require repeated treatment.
Each of these approaches, surgical ablation, electrical stimulation and infusion of pharmacological GABA agonists is effective in disease palliation, but each has significant adverse effects. For example, extensive invasive surgery, a high risk of infection and potential damage to the brain and in the case of drug infusion, very transient efficiency.
Thus, the treatments for neurodegenerative disorders are palliative at best, with limited and transient efficacy. Therefore, a need exists for a therapeutic approach which has advantages in targeting specificity, both short and long-term efficacy, as well as neuroprotection, without extensive surgery or side-effects.